1. Field of the Invention
Embodiments of the present invention generally relate to catalytic gasification of coal.
2. Description of the Related Art
Gasification is the core of a coal-based integrated gasification combined cycle (IGCC) system, which has been considered a better way to use coal for electricity generation due to the adverse environmental impact of CO2. Coal gasifiers are typically fed coal, steam, and oxygen or air. Volatiles are released from coal, leaving a solid char. Combustion of the volatiles and, to a lesser extent, the solid char, rapidly consumes the oxygen. The remaining char is then consumed by the relatively slow steam and CO2 gasification reactions:C(s)+H2O(g)→CO(g)+H2(g) ΔHorxn=+131.46 kJ/gmole  (R1)C(s)+CO2(g)→2CO(g) ΔHorxn=+172.67 kJ/gmole  (R2)
The rate of these two reactions, especially the faster steam gasification reaction (R1) determines the overall gasification rate, or, in other words, the rate that coal can be fed to a gasifier.
Both of the gasification reactions are endothermic. The heat of reaction is provided by the exothermic combustion reactions. The oxygen demand is determined largely by the heat required for these two reactions, plus the heat required to raise the feedstocks to the gasifier operating temperature. Char gasification can be catalyzed by alkaline metal salts. Kapteijin, et al., for example, showed the catalytic effect of 10 group I and group II metals on the carbon dioxide gasification of Norit, an acid-washed peat char. Lang and Neavel showed that a Wyoming sub-bituminous coal, when washed with acetic acid, had a much lower steam gasification rate than when the same coal was unwashed, thereby demonstrating the catalytic effect of naturally occurring alkaline metals.
The addition of a catalyst to coal could allow a lower gasification temperature. This would improve the thermodynamic efficiency of gasification and reduce oxygen demand. Alternatively, at a fixed temperature, a catalyst would allow a higher coal feed rate. Both approaches have the potential to improve the economics of gasification.
A key problem in the use of gasification catalysts is that the spent catalyst leaves the gasifier mixed with ash or slag. If the catalyst is water soluble, then the ash could be leached to recover the catalyst. This adds cost and complexity to the gasification process. Also, catalyst recovery would be less than complete. Catalyst recovery is not feasible if a slagging gasifier is used. Sharma, et al. avoided the ash/catalyst separation problem by gasifying an ash-free Hypercoal, produced by solvent extraction of coal.
Sodium is one of the few catalysts that is both effective and sufficiently inexpensive to be used on a once-through basis, thereby avoiding the need to recover the catalyst. Quyn, et al. showed that sodium chloride is an effective catalyst for the gasification of Loy Yang brown coal. Sodium chloride occurs naturally in this coal. For most gasifiers, however, this is an undesirable form of sodium, because of the corrosive nature of the chloride ion.
Accordingly, there is a need for catalytic gasification of coal using an inexpensive form of sodium produced in large volumes from naturally occurring trona (Na2CO3.NaHCO3) ores.